Treball Final de Grau - UBdiposit.ub.edu/dspace/bitstream/2445/138019/1/TFG_QU Serra Ventura,...

Tutor/s

Dr. José F. García Departament d’Enginyeria Química i

Química Analítica

Dr. Alex Tarancón Departament d’Enginyeria Química i

Química Analítica

Treball Final de Grau

Determination of the radioactive potassium content in bananas.

Determinació del contingut de potassi radioactiu dels plàtans.

Joan Serra Ventura June 2019

Aquesta obra esta subjecta a la llicència de: Reconeixement–NoComercial-SenseObraDerivada

http://creativecommons.org/licenses/by-nc-nd/3.0/es/

I am among those who think that science has great beauty.

Marie Curie

Després d’aquests quatre mesos d’intensa feina i plenament integrat en el món de la

recerca, m’agradaria agrair a totes aquelles persones que han fet possible tirar endavant

aquest treball.

Primerament, agrair els meus tutors, José F. García i Alex Tarancón, per la seva valuosa

ajuda, paciència i per haver-me proporcionat totes les eines i coneixements necessaris per

poder dur a terme aquest treball. També agrair a l’Héctor Bagán per la seva gran implicació i

hores dedicades durant aquest temps.

Agrair també el suport incondicional que m’ha donat la meva mare, el meu pare i la meva

germana durant tots aquests anys, els seus consells i la seva comprensió en els moments més

difícils.

També voldria agrair els companys del grup QÜESTRAM R, tant estudiants de TFG com de

Màster, per totes aquelles tardes amenes que hem passat al laboratori.

Finalment, agrair tots els bons amics que he fet durant el grau, i que de ben segur seran per

tota la vida, per tots els moments i experiències viscudes que han fet d’aquests quatre anys una

de les millors etapes de la meva vida.

A tots vosaltres,

Moltes gràcies.

REPORT

Determination of the radioactive potassium content in bananas. 1

CONTENTS

1. SUMMARY 3

2. RESUM 5

3. INTRODUCTION 7

3.1. Overview in fruit consumption 7

3.2. Banana mineral content 7

3.3. Radioactivity 8

3.3.1. Radiation sources 8

3.3.1.1. Natural radiation sources 8

3.3.1.2. Artificial radiation sources 8

3.3.2. Radioactive decay processes 9

3.3.2.1. Alpha decay (α) 9

3.3.2.2. Beta decay (β) 9

3.3.2.3. Gamma decay (γ) 10

3.4. Potassium-40 10

3.4.1. Properties and interesting aspects 10

3.4.2. Decay processes 11

3.5. Radiation detection and measurement techniques 12

3.5.1. Liquid Scintillation Counting (LSC) 12

3.5.1.1. Scintillation detector 12

3.5.1.2. Mechanism of liquid scintillation 12

3.5.1.3. Quenching processes 13

3.5.1.4. Calibration 13

3.5.2. High Resolution Gamma Spectroscopy (HRGS) 14

3.5.2.1. Semiconductor detector 14

3.5.3. Geiger-Müller Counting (GMC) 14

3.5.3.1. Gaseous ionization detector 14

2 Serra Ventura, Joan

3.6. Dissemination and curiosities 15

4. OBJECTIVES 16

5. EXPERIMENTAL SECTION 17

5.1. Reagents and materials 17

5.2. Apparatus 17

5.3. Banana sample treatment 17

5.4. Total potassium determination 19

5.5. Beta detection 19

5.6. Gamma detection 20

5.6.1. Geometries preparation 20

5.6.2. HRGS measurements 21

5.6.3. GMC measurements 21

6. RESULTS AND DISCUSSION 22

6.1. Ash content determination 22

6.2. Total potassium determination by atomic emission 22

6.3. 40K determination by beta emission 23

6.4. 40K determination by gamma emission 26

6.5. Simple procedure for 40K determination 31

6.6. Radiochemical techniques comparison 34

6.7. Banana dose in humans 35

7. CONCLUSIONS 36

8. REFERENCES AND NOTES 37

9. ACRONYMS 39

APPENDICES 41

Appendix 1: Diagram of the work 43

Appendix 2: LSC calibration with CH3NO2 45

Appendix 3: Homogeneity test 47


1. SUMMARY

Bananas are known to be a great source of potassium. One of the natural isotopes of

potassium, potassium-40, turns out to be radioactive. This isotope is classified as a beta and

gamma emitter and even though it only represents a 0.0117% of the total natural potassium, its

presence in bananas makes this fruit to be classified as a radioactive food.

For the determination of bananas potassium-40 content different radiochemical techniques

have been used depending on the nature of the radioactive emission that is desired to measure.

In one hand, the liquid scintillation technique for the beta particles emission and in the other

hand the high resolution gamma spectrometry for the gamma rays emission. Furthermore, it has

been used a Geiger-Müller counter, whose typical use is to quickly detect radiation in working

surfaces with the aim of establishing a simple 40K determination method in bananas to apply in

teaching environments.

The determination of 40K via high resolution gamma spectrometry in a banana ashes matrix

and a reduced size geometry has provided the most exact results among all the others

radiochemical techniques that have been used in this work. The design and application of a

simple procedure for the same determination have been successfully achieved, having been

banana ashes measurements with a Geiger-Müller counter the best option.

The determination has led to the establishment of the banana activity and the radiation dose

received when this fruit is ingested. In the same way, such data have been used for the

comparison of daily or extreme scenarios, such as tobacco use or a nuclear plant accident, with

the emitted radiation by a single banana for the purpose of raise awareness of the continuous

radiation that our bodies are subjected.

Keywords: Bananas, Radioactivity, Potassium, 40K.


2. RESUM

Els plàtans són coneguts per ser una fruita rica en potassi. Un dels isòtops naturals del

potassi, el potassi-40, resulta ser radioactiu. Aquest isòtop és classificat com un emissor de

partícules beta i gamma i, tot i que la seva abundància sigui només del 0.0117%, la seva

presència en el plàtan és la causa de que aquest sigui denominat com a un aliment radioactiu.

Per a la determinació del contingut de potassi-40 en plàtans s’han emprat diverses

tècniques radioquímiques depenent del tipus d’emissió radioactiva que s’ha volgut mesurar. Per

una banda, la tècnica del centelleig líquid per a l’emissió de partícules beta i, per l’altra banda,

l’espectrometria gamma d’alta resolució per a l’emissió de raigs gamma. També s’ha utilitzat un

altre tipus de comptador anomenat Geiger-Müller, comunament utilitzat per a una ràpida

detecció de radiació en superfícies de treball, amb el propòsit d’establir un mètode simple de

determinació de 40K en plàtans per a la seva futura implementació en entorns docents.

La determinació de 40K mitjançant la tècnica d’espectrometria gamma d’alta resolució en

una matriu de cendres de plàtan i en una geometria de mida reduïda ha proporcionat els

resultats més exactes d’entre totes les tècniques radioquímiques que s’han emprat en aquest

treball. La posada a punt d’un mètode simple per a la mateixa determinació s’ha dut a terme

satisfactòriament, havent sigut la millor opció les mesures de cendres de plàtan amb un

comptador Geiger-Müller.

Aquesta determinació ha portat a l’establiment de l’activitat del plàtan i la dosi de radiació

que rep un ésser humà al ingerir-lo. Tanmateix, també s’ha utilitzat aquesta dada per a la

comparació d’escenaris tan quotidians, per exemple el consum de tabac, com extrems, com ara

un accident nuclear, amb la radiació que emet un plàtan per tal de conscienciar a la població

sobre la contínua exposició a la radiació a la que els nostres cossos estan sotmesos.

Paraules clau: Plàtans, Radioactivitat, Potassi, 40K.


3. INTRODUCTION

It’s worthwhile to start this work with a preliminary contextualization of the topic at different

levels: the day-to-day presence of the paper main protagonist, banana; its role and standing in

our nutrition, its close relationship with radioactivity and how to measure it and finally certain

curiosities that may be of interest for the reader.

3.1. OVERVIEW IN FRUIT CONSUMPTION

Fruits and vegetables are essential components in a healthy diet, also an enough daily

consumption could contribute in the prevention of important diseases such as diabetes, certain

types of cancer and cardiovascular diseases. Approximately 3.9 million of deaths worldwide in

2017 could be connected to a low fruit and vegetable consumption, as announced by World

Health Organization (WHO) and Food and Agriculture Organization of the United Nations

(FAO)(1).

It is well known the recommendation of health

professionals and organizations about the consumption of 5

pieces of fruit or the equivalent of, at least, 400 g of fruit per

day. A large number of reports and surveys about the Spanish

population fruit consumption(2) reveal that fruit predominates

above lactic products and sweets as desserts in daily meals,

resulting that Canarias banana is almost always the most

cheered fruit (Figure 1).

3.2. BANANA MINERAL CONTENT

The mineral composition of Canarias banana may vary according to different factors

referred to all the stages between cultivation and consumption of the fruit: from the geography of

the crops zone, including soil type or altitude; the agriculture techniques used and the

transporting and storage of the product(3). All this parameters could be subject of chemometrics

studies with the aim of classifying unknown banana samples into concrete defined groups.

Figure 1. Canarias banana.


Banana is considered one of the greatest sources of potassium in fruits, only underneath a

few tropical fruits such as avocado, guava or dates(4). Potassium is responsible for several

biochemical functions in human body. Hormone secretion and ion transport through the famous

sodium-potassium pump and potassium channels are a few examples.

3.3. RADIOACTIVITY

It is crucial for understanding the basis of this work to make an overview of radioactivity in

terms of its origin, as well as the mechanisms of its behaviour.

Radioactivity is the decay process of an unstable atomic nucleus by emitting nuclear

particles, such as alpha and beta particles, or gamma rays (in other words, electromagnetic

radiation), to a more stable atomic nucleus.

3.3.1. Radiation sources

Depending on whether there’s human intervention or not, we can distinguish two types of

radiation sources: natural sources and artificial sources(5).

3.3.1.1. Natural radiation sources

Within natural radiation sources, we can differentiate between cosmic radiation and

terrestrial radiation.

In one hand, cosmic radiation can be described as the radiation emitted in outer space that

travels long distances and is capable of reaching the Earth. Its interaction with elements present

in the atmosphere brings out the so-called cosmogenic nuclides, such as 3H and 14C.

In the other hand, terrestrial radiation is defined as the one caused by mineral constituents

present in the Earth since its formation: the primordial nuclides and its descendants. It comes

from the three natural decay chains of 235U, 238U and 232Th. A primordial nuclide that does not

participate in one of the explained natural decay chains is 40K.

3.3.1.2. Artificial radiation sources

Artificial radiation can be described as the radiation caused by nuclear disintegrations and

radionuclides produced in devices created by humans, such as nuclear reactors as the most

evident example.


Some common products of this activities that can end up in the nature by leakages or

controlled releases are: 238U, 235U and 90Sr, ordered from most to least half-life.

3.3.2. Radioactive decay processes

The decay process undergone by a particular nuclide depends on whether exists a

proton/neutron excess or deficit, as well as the mass-energy relation of the initial and final

nuclide.

It can be distinguished three types of radioactive decays: alpha decay, beta decay (positive

or negative), and gamma decay(5).

3.3.2.1. Alpha decay (α)

A nuclide disintegrates emitting an helium-4 particle ( 𝐻𝑒2+)24 of a characteristic energy that

depends on the initial nuclide. The decay can be represented as:

𝑋 → 𝑌 + 𝐻𝑒2+24

𝑍−2𝐴−4

𝑍𝐴

It is common in nuclides with a mass number (A) bigger than 150. Among its characteristics,

it can be highlighted its high ionization power but low penetration effect. Thus, alpha radiation

can be stopped by a simple sheet of paper.

3.3.2.2. Beta decay (β)

A nuclide disintegrates emitting either an electron (beta negative, β-) or a positron (beta

positive, β+). The decays can be represented as:

𝛽−: 𝑋 → 𝑌 + 𝑒− + �̅�𝑍+1𝐴

𝑍𝐴

𝛽+: 𝑋 → 𝑌′ +𝑍−1𝐴

𝑍𝐴 𝑒+ + 𝑣

where �̅� is an antineutrino and 𝑣 a neutrino. These particles were postulated by Wolfgang E.

Pauli in 1927 in order to comply the conservation law of linear momentum and the total kinetic

moment. Because of the null presence of e- or e+ in a nucleus, these processes can be

explained as the transformation of a neutron into a proton emitting an electron and the

antineutrino (in β- radioactivity) or the transformation of a proton into a neutron emitting a

positron and the corresponding neutrino (in β+ radioactivity).

In a much lower probability, a nuclide can also undergo a process of electron capture (EC),

where a nucleus proton captures an atomic orbital electron emitting a neutron and a neutrino.

The decay proceeds as following:


𝑋 + 𝑒− → 𝑌′ + 𝑣 𝑍−1𝐴

𝑍𝐴

This process is in competence with β+ decay, although the difficulties of an innermost K

shell electron of being in close contact with a proton.

The beta decay process can be undergone by any type of nuclide, including a single proton

or neutron and results in a continuous energy spectrum characteristic of every nuclide. It has a

medium ionization power and penetration effect compared to α radioactivity. As a result, a few

millimetres of lead are needed to stop β radioactivity.

3.3.2.3. Gamma decay (γ)

As a consequence of one of the radioactive decay processes explained above, a nuclide

can be left in an excited state. This nuclide can return to the ground state or a lower excited

state by photon emission (γ-ray, electromagnetic radiation) as shown hereunder:

𝑋∗ → 𝑋 + 𝛾𝑍𝐴

𝑍𝐴

The photons energy will depend in the state the nuclide returns. Since the energy states of a

nuclide are discrete (quantized) and characteristic of itself, each nuclide will emit monoenergetic

photons corresponding to its specific energetic transitions.

Finally, these photons are low ionizing but have a high penetration power and many

centimetres of lead are required to stop them.

3.4. POTASSIUM-40

As previously announced, 40K and its content in Canarias banana is the centre of this work,

so it is of interest to know about the nuclide, its radioactive properties and the different decay

processes that can undergo.

3.4.1. Properties and interesting aspects

40K is one of the three natural isotopes (along with 39K with a 93.26% abundance and 41K

with a 6.73% abundance) from a total of 24 potassium isotopes that are currently known(6). It

can be found mostly in the earth crust or forming part of minerals, in addition to its high

presence in oceans water.


It is a radioactive (unstable) isotope that only represents a 0.0117% of the total natural

potassium. With one more neutron but the same atomic number (Z=19) in comparison with the

common potassium, it results in an atomic weight of 39.96 u(6).

A half-life of 1.25·109 years (1.3 billion years, in the order of the age of the universe) makes

40K a source of concern in terms of environmental care and radioactive pollution. One must take

into account that it is not the only radioactive isotope of potassium, but short half-lives (ranging

from milliseconds to less than a day) of these other nuclides and their artificial origin make them

not considered in a study of bananas radiation.

3.4.2. Decay processes

The two types of decay processes that 40K can undergo with a high probability are beta

negative decay and electron capture (Figure 2).

With a 89.25% of probability,

potassium-40 disintegrates by a

β- process emitting a beta

negative particle (electron) with

a maximum energy of 1.311

MeV and an antineutrino. As a

result, 40K decays to the ground

state of calcium-40 (40Ca)(7).

With a 10.55% of probability,

40K disintegrates by electron

capture emitting a neutrino and

followed by gamma emission

(photon) of 1.460 MeV. As a

result, 40K decays to an excited state of 40Ar(7).

In a very small probability, 40K can also disintegrate by electron capture to the ground state

of 40Ar (0.20%) and by beta positive emission (0.001%)(7).

Figure 2. Potassium-40 decay chain.

(Image extracted from M.-M. Bé et al. Table of Radionuclides, 2010, 5, 7-12)


3.5. RADIATION DETECTION AND MEASUREMENT TECHNIQUES

The radiochemical techniques used in this work for the determination of 40K and its

theoretical background are explained hereunder.

3.5.1. Liquid Scintillation Counting (LSC)

Developed in the late 40’s, nowadays liquid scintillation counting (LSC) is the baseline

technique for beta emitters measure, as in 40K case.

3.5.1.1. Scintillation detector

The LSC is based in a scintillation detector, which measures the induced luminescence in a

given compound caused by the electronic transitions to excited states and the subsequent de-

excitation by photon emission.

The scintillating substances used must comply the following requirements: to be transparent

in front of its own light flashes, in order to enable them to reach the photomultipliers; to emit light

with a similar spectrum of that corresponding to the photomultiplier spectral sensitivity curve

(typically between UV and IR zones, with a 400 nm maximum) and finally to emit a light flash

that quickly decays in order to not letting detection pulses to stack, so half-life of excited states

have to be short(5).

It can be distinguished two types of scintillating substances: inorganic and organic, the latter

being the one used in this work.

Organic scintillating substances are able in solid and liquid state (the last ones, usually

formed by dissolution of a solid scintillator in an organic aromatic solvent). Radiation excites the

organic scintillator with subsequent photons emission, which reach the detector.

3.5.1.2. Mechanism of liquid scintillation

Radiation excites organic aromatic solvent molecules, transferring the energy through them,

until scintillating substances (primary scintillators) are reached: the primary scintillator de-

excitation is followed by photon emission, particularly in the UV zone. Usually there are another

scintillating substances (secondary scintillators) that capture the emitted light and re-emit it in a

higher wavelength, which is more appropriate for the photomultiplier spectral sensitivity

curve(8).


The photon avalanches reach the photomultipliers where are amplified, collected and

converted into electric signals. Later, they are converted into tension pulses and lastly counted

and classified into a multichannel according to their amplitudes which in turn is directly related

with the energy of the particle emitted in one nuclide disintegration (Figure 3).

Figure 3. Detection scheme of a nuclear particle in LSC and the corresponding quench processes that can

occur.

(Image extracted from Bagán H. Doctoral thesis, 2011, 28)

3.5.1.3. Quenching processes

During the energy transmission through the solvent, the scintillator and the photomultiplier it

can take place a reduction or inhibition of light emission due to an interference process:

quenching(9). When this phenomenon occurs, it can be expected a reduction in the detection

efficiency of the method. Hereafter are explained a few quenching processes to take into

consideration.

In a colour quenching process, a substance absorbs emitted photons preventing them from

reaching the photomultiplier.

In a chemical quenching process, a substance affects the energy transfer from the organic

aromatic solvent to the scintillator, capturing π electrons from the solvent.

3.5.1.4. Calibration

One of the most important parameters in liquid scintillation is detection efficiency (EffD),

EffD=Net cpm

Act×100

the fraction of photon avalanches counted by the detection system in relation to the total

number of disintegrations produced, where “Net cpm” is the difference between the registered

and the background counts and “Act” corresponds to the activity of the substance.


The most common calibration in LSC is the external calibration curve method, where

detection efficiency is correlated with a parameter, the quenching parameter, obtained either

from the sample spectrum or an external source.

The quenching parameters of an external source is obtained from the sample irradiation by

a radioactive source (the most common is 152Eu). The most used is the Spectral Quench

Parameter of the External Standard, or SQP(E), which is an indicator of the uppermost channel

number that comprises the integration of the 99.5% of the counts produced by the external

source when it irradiates a particular sample(10).

3.5.2. High Resolution Gamma Spectroscopy (HRGS)

In the spectroscopy field of gamma radiation semiconductor detectors are typically used at

the expense of inorganic scintillator detectors such as sodium iodide doped with thallium.

3.5.2.1. Semiconductor detector

In semiconductor detectors, the passage of radiation produces electron-hole pairs with their

subsequent movement, which generates an electric current proportional to the deposited energy

by the nuclear particle.

They are preferred for providing a high resolution that enables the interpretation of complex

gamma spectrum containing various peaks. However, they offer a lower detection efficiency

compared to an inorganic scintillator detector(5).

For its calibration, it has to be taken into consideration that detection efficiency will strongly

depend on the geometry of the sample recipient and the sample matrix.

3.5.3. Geiger-Müller Counting (GMC)

Geiger-Müller detectors are gaseous ionization detectors that work in a particular zone of

polarization.

For a better understanding of these detectors behavior, and concretely the Geiger-Müller

detector, it will be done a short overview of the most important theoretical concepts.

3.5.3.1. Gaseous ionization detector

Gaseous ionization detector measures the ionization produced by a radioactive source to a

gas enclosed in a chamber, which contains two electrodes working as a condenser when a


voltage is applied. The constant voltage will experiment a slight fall, that will later be measured,

due to the electron-ion pairs formed during the ionization process(5).

Depending on the magnitude of the electric field intensity, the detector can work in several

different ways. The Geiger-Müller detector works in the so-called Geiger zone, where the

electric field intensity is such that it produces an electron avalanche due to a massive formation

of electron-ion pairs. As a result, Geiger-Müller detectors cannot distinguish between charged

particles or different energies.

As in the case of HRGS, for GMC calibration has to be taken into consideration that

detection efficiency will strongly depend on the sample matrix and geometry used.

3.6. DISSEMINATION AND CURIOSITIES

It may be of interest to know a few facts and curious comparisons from the world of

radioactivity involving bananas that will surprise the reader.

First of all, the existence of a defined magnitude named banana equivalent dose (BED)

which represents the radiation dose received when a single banana is eaten. This particular

magnitude was established and it is currently used to concern population about the risk of

radiation exposure with a simple comparison with such a common fruit as is banana. For

example, a chest computerized axial tomography (CAT) scan done in a hospital would

correspond to an ingest of 70.000 bananas (7 mSv)(11). Moreover, it can be interesting for

those smoker readers to know that a 1 pack of cigarettes per day habit during a whole year

equates to approximately 700.000 BED (70 mSv).

A frequently asked question when talking about the radioactivity within bananas is whether

its consumption is harmful to health. It will be later answered with the results obtained in this

work.


4. OBJECTIVES

The main objective of this work is the development of a procedure to determine 40K in

bananas by radiochemical techniques. To achieve it, the following points are proposed.

First of all, the elaboration of a sample treatment plan with the aim of eliminate the

interfering substances in bananas to obtain a suitable matrix to work on the further

determinations.

Secondly, the total potassium determination of bananas by a spectroscopic technique such

as flame emission. The following results will be referenced to the value obtained in this stage of

the work.

Next, the design and application of a procedure for the 40K determination of bananas by its

beta and gamma emissions.

The comparison of 40K activities obtained by the different radiochemical techniques to

discuss possible differences obtained.

Finally, the design and application of a simple procedure for the 40K determination to

implement in a learning environment.


5. EXPERIMENTAL SECTION

5.1. REAGENTS AND MATERIALS

All the reagents used are of analytical grade. Hydrochloric acid fuming 37% by Merck KGaA

(Darmstadt, Germany) and nitric acid 65% by Fisher Scientific (Loughborough, England) have

been used to prepare the acid solution for ashes attack. Nitric acid and nitromethane (by Merck

KGaA) have been used as chemical quenching agents.

For LS measurements, the scintillation cocktail Optiphase HiSafe 3 by PerkinElmer has

been used.

5.2. APPARATUS

An atomic absorption/emission spectrometer AAnalyst 200 AA

(by PerkinElmer) has been used for total K measurements.

A 1220 Quantulus liquid scintillation detector (by EG&G Wallac)

with logarithmic amplification and a multichannel analyser of 4096

channels has been used for LS measurements (Figure 4).

A high resolution gamma spectrometer with an intrinsic

germanium detector (by Canberra) has been used for gamma

emissions measurements.

A LB 124 Geiger-Müller counter (by Berthold Technologies) has

been used for β/γ measurements.

5.3. BANANA SAMPLE TREATMENT

A representative sampling of bananas and its treatment by crushing and dry ashes

method(12) previously designed (Table 1) has been done for the later determinations of total K

and 40K. The sample used during all the work has been the edible parts of Canarias banana

from Plátano de Canarias brand, Cavendish variety with a minimum size of 16 cm.

Figure 4. 1220 Quantulus liquid scintillation detector by

EG&G Wallac.


Stage Image Description

Raw material

Banana edible part

Crushed

Homogeneous paste after crushing

Carbonized

Dark brown residue after 200ºC sand bath

Calcined

White ashes after 500ºC calcination

Acid attack

Ashes dissolution. Yellow coloration is due to HNO3 thermal

decomposition

Table 1. Sample treatment stages.


Bananas have been crushed in a closed blender until achieving an homogeneous paste. 30

g of the result have been transferred into porcelain capsules. The replicates have been first

heated in a sand bath at approximately 200ºC and later calcined at 500ºC, performing a ramp of

temperatures to avoid sample inflammation. During the process temperature has not been

increased beyond 500ºC to prevent potassium loss(3)(13). The obtained ashes have been

dissolved with 25 mL of a HCl 1:1/HNO3 1:1 solution and an additional 5 mL of concentrated

HNO3. To ensure the full dissolution, capsules have been heated to approximately 100ºC in a

sand bath.

5.4. TOTAL POTASSIUM DETERMINATION

Total potassium determination of bananas has been done by Flame Atomic Emission

Spectroscopy (FAES) with few additional sample treatment steps.

Capsule contents have been filtered with ashless 110-mm filter papers (Whatman,

Buckinghamshire, UK) and transferred into 250 mL flasks filled up with double deionized

water(14). A volume of 2.5 mL of the solutions has been diluted to 25 mL flasks in order to

reach the concentration of the technique calibration range. A concentration of 1000 ppm Na has

been introduced into the measuring 25 mL flasks acting as a ionization suppressor(15).

Standards have been prepared by consecutive dilutions from a 1000 ppm K solution into a

10-80 ppm K range, containing a 1000 ppm Na concentration(15).

Blank solutions have been prepared with 2.5 mL of HCl 1:1/HNO3 1:1 solution, a 1000 ppm

Na concentration and filled up with double deionized water.

Measurements have been performed at the emission wavelength of 776.5 nm(15)(16).

5.5. BETA DETECTION

For 40K determination by beta emissions, LSC with a calibration method by chemical

quenching, using HNO3 and CH3NO2, has been descripted and tested for banana ashes

solution matrix.

LS sample vials have been prepared by adding 6 mL of 250 mL flasks prepared in Section

5.4. and filled up with 14 mL of scintillation cocktail(17) (Figure 5). Each sample vial has been

measured for 5 h with 20 min of SQP(E).


LS standard vials for chemical quenching calibration have been prepared with increasing

amounts of HNO3 or CH3NO2 (0-1000 μL and 0-25 μL ranges, respectively), 1 mL of a 0.3 g

KCl/mL solution, 14 mL of scintillation cocktail and

filled up with double deionized water to 20 mL.

Each standard vial has been measured for 2 h with

20 min of SQP(E).

LS blank vials have been prepared by adding

increasing amounts of HNO3 or CH3NO2 (same

ranges than standard vials), 14 mL of scintillation

cocktail and filled up with double deionized water.

Each blank vial has been measured for 5 h with 20 min of SQP(E).

5.6. GAMMA DETECTION

HRGS and GM measurements for 40K determination have been done for crushed banana

and ashes matrices.

5.6.1. Geometries preparation

A sample geometry and a sample geometry spiked with KCl have been prepared for

detection efficiency calibration of the techniques.

For crushed banana geometries (GA1 and GB1), two 500 mL

geometries (Figure 6) have been filled achieving the same weight

and height of sample inside the recipient. GA1 only contained

crushed banana and GB1 has been prepared by adding

consecutive layers of crushed banana and dried KCl (a total of

approximately 30 g) with the aim of obtaining an

homogeneous sample.

For the preparation of banana ashes geometries (GA2

and GB2), previous geometry contents have been treated

as indicated in Section 5.3. and ashes obtained have been

disposed in optimised geometries (100 mL geometry

covers, Figure 7), achieving the same weight and height,

Figure 6. On the left, GA1. On the right, GB1.

Figure 7. On the left, GA2 geometry. On the right, GB2 geometry.

Figure 5. LS measured vials and 250 mL flasks prepared in Section 5.4.


which have been covered with parafilm. GA2 and GB2 have been prepared from GA1 and GB1,

respectively (Appendix 1).

5.6.2. HRGS measurements

GA1 and GA2 geometries have been measured for 3 days. GB1 and GB2 geometries have

been measured for 1 day due to the higher activity caused by the KCl addition in both

geometries.

Detection efficiency calibration of the instrument, as a function of the gamma decays ratio

and its energies, has also been done by Servei d’Anàlisis Isotòpiques (Universitat de

Barcelona).

5.6.3. GMC measurements

Geometries have been placed in the centre of the GMC detection surface and measured for

5 h each (Figure 8). In the case of crushed

banana matrix, measurements have been

performed with and without the geometry cover

and an alternative rectangular geometry with a

bigger surface have been tested.

Background radiation measurements have

been performed with an empty 500 mL geometry

and an empty optimised geometry for crushed

banana and ashes matrices, respectively.

Figure 8. On the left, β/γ counts per second are shown in the GM screen. On the right, set up

for crushed banana geometries measurement.


6. RESULTS AND DISCUSSION

In the next sections results obtained for ash content determination, total potassium

determination by atomic emission measures, 40K determination by beta emission measures by

LSC and by gamma emission measures by HRGS are discussed, in addition to the results

obtained for the simple procedure designed for 40K determination by gamma emission using

GMC.

6.1. ASH CONTENT DETERMINATION

As a previous step to the total K and 40K determinations, the banana ash content has been

determined and a suitable matrix has been achieved, in addition to the 40K preconcentration.

For ash obtainment, a calcination step of the sample at 500ºC has been performed in order

to eliminate the organic matter and humidity within bananas (Table 2).

Capsule Crushed banana mass (g)

Banana ashes mass (g)

Ash content (%)

RE1 30.15 0.3072 1.02

RE2 30.05 0.3280 1.09

RE3 30.06 0.3120 1.04

Table 2. Crushed banana mass and banana ashes mass obtained for ash content determination. The replicates has been used for the posterior FAES and LSC analyses.

A significant reduction in sample dimension has been observed (Table 2) after calcination

mostly due to the high humidity content of bananas(18), mainly remaining metal oxides and

carbonates, resulting in an average ash content of 1.04% (0.03) (RSD=3%).

6.2. TOTAL POTASSIUM DETERMINATION BY ATOMIC EMISSION

For the later comparison of the different techniques used in this work, total potassium

content of bananas has been determined by FAES.


The 776.5 nm emission spectral line has been chosen in a range of concentrations from 10

to 80 ppm K for the sample and standards.

For the technique calibration an external calibration curve has been obtained with a linear

fit, that follows the equation

𝑦 = 404.527𝑥 + 6860.77 𝑅2 = 0.9662

Table 3 shows the total K concentration per 100 g of banana calculated for the samples

prepared.

Sample [K] (mg/L) Emission (a.u.)

[K]/100 g banana (mg/100 g)

1 60.21 31218 494.3

2 60.87 31483 504.6

3 58.08 30355 487.7

Table 3. Potassium concentration, emission energy and potassium concentration per 100 g of banana of samples prepared.

The total K concentration per 100 g of banana calculated has been 496 mg K/100 g (9)

(RSD=2%).

The total K concentration has been compared with data found on bibliography about Plátano

de Canarias, which has been 385 mg K/100 g banana(19). Differences has been observed,

probably caused by the differences between banana crops (Section 3.2.), but results are in the

same order of magnitude. Thus, FAES result obtained has been taken as the reference value

for comparisons throughout the posterior determinations.

6.3. 40K DETERMINATION BY BETA EMISSION

At this point, the 40K content in bananas has been determined by its beta emissions using

LSC.

The technique calibration has been first performed with the addition of increasing volumes

of HNO3 in the measuring vials, which is present in the samples, in order to correct the possible

quenching processes occurring in the matrix. Due to the results obtained, a calibration with

CH3NO2, which is known to be a chemical quenching agent(20), has been performed.

Figure 9 shows the SQP(E) values of standards proportioned by the instrument and the

corresponding detection efficiencies calculated for the calibration with HNO3.


88

88,5

89

89,5

90

90,5

91

720 740 760 780 800 820De

tect

ion

eff

icie

ncy

(%

)

SQP(E)

SQP(E) vs. Detection efficiency (%) HNO3

quenching calibration, LSC

Even though it had been thought that HNO3 could cause colour quenching by its thermal

decomposition to NO2 or by nitration of the scintillation vial plastic, no yellow colour has been

observed in the samples. The calibration curve has shown a constant trend in detection

efficiency of 89.2% (0.7) (RSD=0.8%) regardless the different SQP(E) values, situated in a

range of 731 to 814, which indicates that HNO3 has been acting as a chemical quenching agent.

Thus, a polynomial fit has not been shown.

Figure 10 shows the spectra obtained for standard vials prepared with different HNO3

volumes.

Figure 10. Spectra of the energy in every channel in front of the count rate of the standard vials containing HNO3, from S1 to S9.

Figure 9. SQP(E) quench calibration curve representation for increasing volumes of HNO3.


Even when the quenching causes the shift of the spectra to lower energies (in channels),

caused by the increasing volumes of HNO3 in standards (S1 does not contain HNO3 and S9

contains the maximum HNO3 amount), it has been proved to not be enough to see a loss of

count rate in the low energy part of the spectra (channels from 0 to 100) and hence a decrease

in detection efficiency. It is due to the high energy of the beta particles that 40K emits, most of

them appearing in channels from 600 to 800.

As HNO3 has been proved to not produce such quench to see changes in detection

efficiency, CH3NO2 has been used to calibrate the chemical quenching effects present in the

matrix.

Even though CH3NO2 nitro electronegative group presence, which captures the π electrons

associated with the scintillation cocktail aromatic solvent reducing the energy transfer from the

beta emission, same results has been observed than the ones obtained for HNO3 calibration: a

constant trend in detection efficiency of 91.7% (0.9) (RSD=1%) regardless the different

quenched standards situated in a range of 770 to 815 SQP(E) values. The shift of the spectra to

lower energies for increasing volumes of CH3NO2 is even lower than the one observed in HNO3.

Therefore, the constant detection efficiency of 91.7% obtained by CH3NO2 calibration has been

selected for the posterior samples measuring (Appendix 2).

Figure 11 shows the sample spectra obtained and Table 4 includes the SQP(E) and net

count rate values for each sample vial.

Figure 11. Spectra of the energy in channels in front of the count rate for the sample replicates.


The sample spectra has been situated in the 600-800 channels window and no other

interfering peaks corresponding to different radionuclides has been observed. Even though a

Savitzky-Golay smoothing has been applied, fluctuations in the sample spectra have been

observed due to its count rate order of magnitude in comparison with the one for standards.

Table 4 shows the 40K concentration and the deduced total K concentration in the samples

prepared, as well as the SQP(E) values and the net count rate obtained for each replicate.

Sample SQP(E) Net count rate (cpm)(a)

Activity per 100 g of banana (Bq/100 g)

[K] per 100 g of banana (mg/100 g)

1 796.3 5.181 13.02 411.2

2 803.2 5.119 12.91 407.6

3 805.4 5.337 13.45 424.8

(a) Net count rate has been obtained by subtracting the integrated, average counts obtained of the three blanks prepared from the count rate measured for each sample vial. Average blank count rate = 3.297 cpm. Table 4. Calculated 40K and total K concentrations in the replicates. In addition, SQP(E) and net count rate

values proportioned by 1220 Quantulus have been shown.

The average SQP(E) value obtained for the replicates has been approximately of 802,

inside the calibration range of SQP(E) values obtained, which has indicated a low HNO3

concentration and hence a low quenching effect in the samples.

The 40K activity calculated per 100 g of banana has been 13.1 Bq/100 g (0.3) (RSD=2%)

and the total K concentration per 100 g of banana has been 415 mg/100 g (9) (RSD=2%).

High precision in the 40K determination results have been observed for the LSC

measurements of banana ashes solution. The result obtained for total K concentration per 100 g

of banana deduced from the 40K determination by LSC has been close to the reference value of

496 mg/100 g obtained for total K concentration by FAES. Closer results to the total K

concentration obtained by FAES were expected due to the clean matrix used, banana ashes

solution, with no signal attenuation caused by the density component, in addition to the non

geometry dependence of LSC in comparison with the other radiochemical techniques that have

been used in this work.

6.4. 40K DETERMINATION BY GAMMA EMISSION

The second technique that has been tested for the 40K determination has been HRGS. The

results obtained for HRGS measurements of 40K gamma emissions of crushed banana and


banana ashes matrices are shown hereunder along with the technique calibration that has been

performed.

Crushed banana geometries have been prepared by duplicate, one of them spiked with

solid KCl standard for the technique detection efficiency calibration. Dried KCl has been used as

the standard instead of a radioactive 40K standard because the same geometries for HRGS

measurements have been used for the later simple procedure for 40K determination, whose

objective is to be implemented in teaching environments where radioactive standards presence

is not common. Heterogeneities in the crushed banana matrices, probably caused by

encapsulated air between crushed banana layers, have been observed even though it has been

tried to achieve geometries as much homogeneous as possible (Appendix 3).

After its measurement, a calcination step has been performed over the geometry contents to

obtain the next ashes geometries, in order to minimise matrix effects.

Gamma spectrum of crushed banana (GA1) and crushed banana spiked with KCl (GB1)

geometries obtained are shown in Figure 12 and 13, where gamma emission energies have

been represented in front of the count rate registered (pulses/min).

200 400 600 800 1000 1200 1400 1600 18000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

energia (keV)

imp/m

in

< 0

1 <

02

< 0

3

< 0

4 <

05

< 0

6

< 0

7

< 0

8 <

09

< 1

0

< 1

1-K .

40b

< 1

2 <

13

Figure 12. Representation of the gamma emissions of GA1 geometry containing crushed banana.


200 400 600 800 1000 1200 1400 1600 18000

0.5

1

1.5

2

2.5

3

3.5

4

energia (keV)

imp/m

in

< 0

1

< 0

2

< 0

3

< 0

4-K .

40b

< 0

5

Figure 13. Representation of the gamma emissions of GB1 geometry containing crushed banana and spiked with KCl.

The gamma emissions corresponding to the 40K decay to the 1460 keV excited state of 40Ar

by electron capture have been observed in the spectra at 1460.75 keV in both cases, giving a

higher net count rate in the case of GB1: 2.78·10-1 pulses/s in front of the 4.67·10-2 pulses/s of

GA1 and a total activity of 592 Bq for GB1 in front of the total activity of 99 Bq corresponding to

GA1, all due to the KCl mass that has been added into the geometry. Since the technique only

registers the gamma disintegrations, count rates have been clearly lower in comparison with the

ones registered in LSC, where beta negative emissions have been registered, due to the

10.55% probability of gamma emissions in front of the 89.25% probability of beta negative

emissions in the case of 40K.

The second more intense peak that has been observed at 511 keV corresponds to a photon

emission by a positron annihilation, procedure that happens with only a 0.002% probability. This

peak have been only seen in GA1 spectrum due to the intensity relation with the 40K peak. No

more significant peaks corresponding to other natural radionuclides have been observed, hence

the radiation within banana sample has been assumed to be caused only by 40K.

The spiked geometry (GB1) has been used for the detection efficiency calibration and

therefore the calculation of the crushed geometry (GA1) 40K content and total K concentration

deduced (Table 5).


Geometry Net count rate (pulses/s) (a)



GA1 4.667·10-2 18.05 570.0

(a) Net count rate has been obtained by subtracting the background contribution to the measure, which turned out to be 0.165 pulses/min.

Table 5. 40K and total K concentration results calculated from HRGS measurements of crushed banana matrix for the crushed banana geometry (GA1). In addition, the net count rate for the geometry has been

shown.

The detection efficiency of the technique has been calibrated for the 500 mL geometry and

the crushed banana matrix, resulting in a value of 0.44%. It has been slightly higher from the

calibrated by Servei d’Anàlisis Isotòpiques (Universitat de Barcelona), whose value has been of

0.40%, due to the lower density of crushed banana matrix in front of water and soil matrices that

they have been used for the geometry calibration procedure.

For HRGS measures of crushed banana, 40K activity calculated per 100 g of banana has

been 18.1 Bq/100 g and the total K concentration deduced from the geometry 40K content has

been 570 mg/100 g.

The 40K activity obtained for the HRGS measurements of the crushed banana geometry has

been of the same order of magnitude and hence comparable with the one obtained with LSC

(13.1 Bq/100 g). Nevertheless, the total K concentration deduced from the 40K content obtained

has been far different from the one obtained by FAES (496 mg K/100 g banana) and even more

for the obtained by LSC (415 mg K/100 g banana), hence has not been comparable.

Differences between techniques have been attributed to crushed banana matrix attenuation of

gamma emissions and geometry size, whose height has not allowed gamma rays from the top

content to be emitted in an angle capable to reach the detection window of the HRGS

germanium detector.

After the crushed banana measures, contents of both crushed banana and crushed banana

spiked with KCl geometries have been calcined and ashes obtained have been placed in

optimised geometries with a much lower height (100 mL geometry cover) protected with a semi-

transparent, flexible film (Parafilm). Differences between ashes coloration have been

attributable to a possible impurity from KCl (Figure 7).

Ashes geometry (GA2) and ashes geometry spiked with KCl (GB2) have come from GA1

and GB1, respectively. As in the previous case, the spiked geometry has been used for the


detection efficiency calibration of the geometry and the ashes matrix and therefore the 40K and

total K concentrations of the ashes geometry (Table 6).

Geometry Net count rate (pulse/s)(a)



GA2 0.1656 13.07 475.1

(a) Net count rate has been obtained by subtracting the background contribution to the measure, which turned out to be 1,207 pulses/min.

Table 6. 40K and total K concentration results calculated from HRGS measurements of banana ashes matrix for the banana ashes geometry (GA2). In addition, the net count rate for the geometry has been

shown.

The calibrated detection efficiency for the optimised geometry and the banana ashes matrix

has turned out to be higher than the one calibrated for the crushed banana geometry,

specifically of 1.75%. It has been attributed to the great reduction in geometry volume, achieving

a lower height and hence having the most part of the sample in close contact with the

germanium detector window, and the change from crushed banana to banana ashes matrix,

which has provided a lower attenuation of the 40K gamma emissions. It has not been compared

with Servei d’Anàlisis Isotòpiques because they do not usually use 100 mL geometry covers as

a geometry for HRGS measurements and therefore they do not have data registered for its

calibration.

Gamma emissions corresponding to 40K have appeared at 1460.75 keV and the net count

rate obtained has been, as expected, higher for the spiked geometry in comparison with the

banana ashes geometry (approximately 90 Bq of GB2 in front of the 63 Bq corresponding to

GA2).

For HRGS measurements of banana ashes, 40K activity calculated per 100 g of banana

have been 13.1 Bq and the total K concentration deduced from the geometry 40K content has

been 475 mg/100 g.

The 40K activity per 100 g of banana obtained for banana ashes measurements using HRGS

has been identical to the one obtained for LSC (13.1 Bq/100 g) and very similar, hence

comparable, to the results obtained for the crushed banana matrix measurements by the same

technique, HRGS (18.1 Bq/100 g). The total K concentration per 100 g of banana deduced has

resulted to be the closest to the one obtained by FAES measurements (496 mg/100 g). This

satisfactory results have been attributed to geometry size reduction and decrease of matrix

attenuation of gamma emissions due to the crushed banana calcination. The whole set of


advantages and precise results obtained have made the gamma emission measurement of

banana ashes by HRGS the best technique for the 40K determination in bananas among the

radiochemical techniques that have been used in this work.

6.5. SIMPLE PROCEDURE FOR 40K DETERMINATION

Finally, the same crushed banana and banana ashes geometries from HRGS

measurements have been measured with a routine use Geiger-Müller Counter (GMC), which is

not specific for radiochemical analyses, in order to establish a simple procedure for 40K

determination in bananas that could be implemented into teaching environments. The detection

efficiency calibration has been done by the same procedure than the one employed for HRGS

measurements: the spiked geometry with KCl has been used for the detection efficiency

calibration of the geometry and the matrix for GMC measurements for subsequently determine

the 40K concentration in the non-spiked geometry.

The results obtained for GMC measurements of 40K gamma emissions of crushed banana

and banana ashes matrices are shown hereunder along with the technique calibration that have

been performed.

Since most of the beta particles do not reach the detection surface because of their

penetration effect, one may assume the count rate obtained is mainly due to gamma emissions

only. Nevertheless, for crushed banana matrix, GMC measurements have been performed with

and without geometry covers to see whether the change causes a filter effect on beta particles.

GMC have proportioned the β/γ count rate of the geometry contents (Table 7). For the filled

geometries, the instrument has only been capable to measure the emissions from the top of the

geometry, concretely the surface of the sample. In the case of background, when measures

have been performed with covers, the GMC has been measuring the plastic material of the

cover, while when measures have been performed without covers the GMC has been

measuring the radionuclides present in the air that have been encapsulated inside the

geometry.


Geometry(a) β/γ count rate (cps)

Background w/o c 9.13

Background w/ c 7.85

GA1 w/o c 8.58

GA1 w/ c -(b)

GB1 w/o c 9.26

GB1 w/ c 8.70

(a) w/o c = measure performed without geometry covers. w/ c = measure performed with geometry covers. (b) Measures for GA1 geometry with cover have not been performed because results obtained for the other geometry measures were

not satisfactory. Table 7. Count rate measured for the background and filled GA1 and GB1 geometries.

The crushed banana spiked geometry (GB1) has given close but higher count rates

compared to the ones for the background radiation, regardless the cover presence or not. The

crushed banana geometry (GA1) has given non satisfactory results: lower count rate than the

background has been obtained for measures without cover and measures with cover have not

been performed because worse results have been expected.

A rectangular 400 mL geometry filled with crushed banana, which has got a bigger surface

for gamma detection, has been tested to improve the results but again same or lower count

rates than the background have been obtained.

The big volume that has represented a crushed banana full filled 500 mL geometry has not

allowed gamma emissions to reach the GMC detection surface. Hence, a reduction of the

geometry size and sample volume has been needed in order to obtain relevant results.

Consequently, calcination of geometry contents has been done for banana ashes

geometries preparation and the posterior GMC measurements. Results of the β/γ count rate

registered and 40K activity calculated, along with the total K concentration deduced from 40K

content, are shown in Table 8.

Geometry β/γ net count rate (cps)(a)



GA2 5.18 11.4 373.7

(a) Net count rate has been obtained by subtracting the background contribution to the measure, which turned out to be 9 cps for an empty optimised geometry covered with Parafilm. Table 8. 40K and total K concentration results calculated from GMC measurements of banana ashes for the geometry content and per 100 g of banana. In addition, the net count rate obtained for the banana ashes

geometry is shown.


Results of the count rate measured for banana ashes (GA2) and banana ashes spiked with

KCl (GB2) geometries, 14.18 cps and 18.09 cps respectively, have been significantly higher

than the one measured for background (9 cps). Thus, in contrast with crushed banana GMC

measurements, it has been possible to use this data for the later activity and total K

calculations.

The calibrated detection efficiency for GMC measures of ashes geometry has turned out to

be surprisingly high, 99.1%, compared with the radiochemical techniques that have been

previously used. Reduction of the geometry size, concretely in height, has caused an increase

of gamma emissions detection due to the closer position of the overall ash mass to the

detection surface of the instrument. In comparison, the overall crushed banana mass had been

disposed along a bigger geometry (500 mL) where gamma emissions from the bottom content

had an emission angle that did not enable its detection. In addition, the decrease in sample

volume and density by calcination of crushed banana to ashes have represented a decrease in

matrix attenuation of the gamma emissions.

For GMC measurements of banana ashes, 40K activity calculated per 100 g of banana have

been 11.4 Bq/100 g and the total K concentration deduced from the 40K content has been 374

mg/100 g.

The 40K activity per 100 g of banana obtained for banana ashes matrix GMC measurements

has been closer to the ones obtained for LSC and HRGS by banana ashes measures (13.1

Bq/100 g in both cases) than the one obtained for crushed banana measures using HRGS, but

equally in the same order of magnitude and hence comparable. Calibration through KCl spiking

has resulted to be satisfactory by giving high precision in the results, regardless the high value

of measure uncertainty associated to the instrument used.

The total K concentration per 100 g of banana deduced has resulted to be far from the

reference value obtained by FAES measurements (496 mg/100) and it has been attributed to

the Geiger-Müller instrument, which task is not to accurately analyse radioactive samples but to

quickly detect radiation in surfaces due to accidental leakages in a daily basis.

Finally, it has been concluded that GMC can be used for 40K determination in bananas by

using a banana ashes matrix to minimise the matrix attenuation effect over gamma emissions

and an optimised geometry to ensure the detection of the maximum number of these emissions.


6.6. RADIOCHEMICAL TECHNIQUES COMPARISON

Finally, a discussion over the radiochemical techniques that have been used in this work

has been done in order to expose the main advantages and disadvantages of each one and

select which suits in different cases.

The main advantages of LSC are the low limit of detection due to the high detection

efficiency that provides in the case of a high-energy beta emitter such as 40K, a low background

thanks to the lead shielding surrounding the detector and a low measure uncertainty that

ensures a high precision between replicates. In contrast, the technique requires an important

sample treatment to achieve a clear matrix without interferences, in addition to large times of

measurement (easily 40 h for a whole set of standards, samples and blanks) and the difficult

access to an ultra low level liquid scintillation spectrometer due to its high cost. Thus, LSC is

appropriate for cases where low activity samples are analysed and instruments for sample

treatment (such as muffles or sand baths) and measure (1220 Quantulus) are available.

For the case of HRGS, the main advantages of the technique are the few steps needed for

sample treatment, allowing to directly measure crushed banana samples obtaining satisfactory

results or calcine to ashes for improving the results exactitude; the high selectivity between

gamma emissions due to its resolution characteristic of a spectrometric technique and its low

background thanks to the shielding surrounding the germanium detector. Instead, it does not

have such low limit of detection as LSC and the time of measurement for the sample and the

calibration geometries can reach up to 4 days. Hence, HRGS is appropriate for cases of routine

measures of samples where sample treatment wants to be skipped for saving time and for not

adding uncertainties associated with the treatment procedure in the determination.

Last, GMC presents the advantage of its simplicity when it comes to use it due to its real

task of routine measures of working surfaces in radioactive installations, in addition to the little

sample treatment needed for the approximate determination of 40K in bananas. In contrast, the

technique offers a high measure uncertainty and a high background, which is caused by the null

presence of a shielding to reduce the cosmic radiation that is detected along with the sample

emissions, in addition to a high limit of detection which forces to have samples with relatively

high activity to obtain significant results. Nevertheless, if the purpose of the determination is to

teach radiochemistry to students without knowledge in this field, the simple procedure described

in this work is entirely valid for 40K determination in bananas.


6.7. BANANA DOSE IN HUMANS

In order to finish this work in the same way it was started, the question of whether the

banana consumption is harmful to human health has been answered.

With the banana activity results obtained by measures of banana samples with different

radiochemical techniques it has been possible to see that an activity of an average 14 Bq/100 g

of banana is not dangerous for humans.

However, an approximation of the effective dose received for a single banana ingest has

been calculated and compared with the dose limits established for the average population, for

radioactive installations workers and also the considered limit dose for death. The effective dose

can be described as the sum of the different weighted equivalent doses in the whole tissues and

organs of human body.

Assuming that 40K radiation is caused by electrons (β- decay) and photons (γ decay) and this

particles mainly affect organs and tissues such as stomach, liver, small intestine, large intestine,

kidney, pancreas, spleen, adrenal glands and bones(21), the effective dose calculated is 3·10-8

mSv.

A dose limit of 20 mSv/year is given to workers from a radioactive installation, while for

members of the public this value decreases to 1 mSv/year. Moreover, an absorbed dose of

1000-2000 mSv can be deadly in 6 to 8 weeks, while the increase of the dose up to 30000 mSv

leads to death in no more than 1 day.

Therefore, with an effective dose of 3·10-8 mSv per banana can be concluded that the

human banana intake is completely harmless to health.


7. CONCLUSIONS

After the discussion of the results that have been obtained for 40K determination by the

different radiochemical techniques used, the following conclusions have been drawn.

A procedure to determine 40K in bananas has been established comprising the sample

treatment of crushed banana and its calcination to ashes, the sample measure by the 40K

gamma emissions using HRGS, equipped with an intrinsic germanium detector, in the format of

banana ashes matrix placed into an optimised geometry, which has been a 100 mL geometry

cover, and the technique calibration by KCl standard spike of a sample geometry.

FAES technique for the reference value obtainment of total K concentration in bananas has

been reinforced to be a robust method with low measure uncertainty for the obtention of

accurate results.

A procedure for 40K determination by beta emissions has been designed and tested using

LSC technique. Accurate results have been obtained by using banana ashes solution with

mainly nitric acid and the subsequent calibration for matrix quenching effects by using

nitromethane, finally establishing a constant detection efficiency. The technique has provided

high precision and the possibility to work with low activity samples, which can be a crucial factor

whether the experiment can not be performed with a big quantity of bananas as it has been in

this case.

Finally, a simple procedure for 40K determination in bananas has been designed and

applied for its future implementation in radiochemistry teaching environments. GMC

measurements of banana ashes in an optimised geometry with a calibration through KCl spike

have given significantly higher count rates than the background for the posterior 40K

concentration calculation, whose result has been quiet satisfactory if the purpose of the

determination is to teach students without radiochemistry knowledge.


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9. ACRONYMS

A Mass number

BED Banana Equivalent Dose

CAT Computerized Axial Tomography

EC Electron Capture

FAES Flame Atomic Emission Spectroscopy

FAO Food and Agriculture Organization of the United Nations

GM Geiger-Müller

GMC Geiger-Müller Counting

HRGS High Resolution Gamma Spectroscopy

IR Infrared

LOD Limit of Detection

LS Liquid Scintillation

LSC Liquid Scintillation Counting

MCA Multichannel Analyser

PMT Photomultiplier Tube

RSD Relative Standard Deviation

SQP(E) Spectral Quench Parameter of the External Standard

UV Ultraviolet

WHO World Health Organization

Z Atomic number


APPENDICES


APPENDIX 1: DIAGRAM OF THE WORK

Figure A1 shows the steps that have been followed in this work, from the initial raw bananas

sample to the 40K determinations by different radiochemical techniques, going through all the

sample treatment procedures in order to achieve a suitable matrix.

Figure A1. Diagram of all the procedures involved in the work.


APPENDIX 2: LSC CALIBRATION WITH CH3NO2

Figure A2 shows the SQP(E) values of standards proportioned by the instrument and the

corresponding detection efficiencies calculated. Quench curve is explained in Section 6.3.

Figure A3 shows the spectra obtained for standard vials prepared with increasing volumes

of CH3NO2, explained in Section 6.3.

90,0

91,0

92,0

93,0

94,0

760 770 780 790 800 810 820

De

tect

ion

eff

icie

ncy

(%

)

SQP(E)

SQP(E) vs. Detection efficiency (%) CH3NO2 quenching curve, LSC

Figure A2. SQP(E) quench calibration curve for increasing volumes of CH3NO2.

Figure A3. Spectra of the energy in every channel in front of the count rate of the standard vials containing CH3NO2, from S1 to S9.


APPENDIX 3: HOMOGENEITY TEST

An homogeneity test over the crushed banana geometry spiked with KCl (GB1) has been

performed to ensure that its preparation by addition of consecutive layers of crushed banana

and KCl has been correctly done, having an homogeneous distribution between crushed

banana and the added KCl to achieve significant results in the posteriors measures by HRGS.

Three 100 mL geometries have been filled up with GB1 content from the top (H1), middle

(H2) and bottom (H3) of the geometry and have been measured by HRGS, each one during 1

day. Results obtained are shown in Table A1.

Geometry Net count rate (pulses/s)(a)

H1 7.82·10-2

H2 1.06·10-1

H3 1.20·10-1

(a) Net count rate has been obtained by subtracting the background contribution to the measure, which turned out to be 1.207 pulses/min.

Table A1. Net count rate obtained for the three geometries of the homogeneity test.

Slightly different net count rates have been obtained for the three homogeneity test

geometries, attributable to negligible bigger amounts of KCl in the middle and bottom of the

geometry. Hence, the crushed banana geometry spiked with KCl (GB1) has been considered to

be homogeneous.

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